Conventionally, catalytic reforming of straight run naphtha with a platinum/alumina catalyst has been employed widely for commercial purposes in order to produce gasoline with a higher octane number or aromatic hydrocarbons. The feedstock naphtha for the catalytic reforming is a fraction with a boiling point of 70 to 180° C. when an automobile gasoline is intended to be produced and a fraction with a boiling point of 60 to 150° C. when aromatic hydrocarbons such as xylene, that is BTX, are intended to be produced. However, it has been difficult to produce high octane number gasolines and aromatic hydrocarbons from a feedstock light hydrocarbon containing mainly hydrocarbons having 7 or fewer carbon atoms at a high yield with the conventional catalytic reforming processes because the conversion rate of the feedstock hydrocarbon to aromatics decreases with a decrease in the carbon atom number of the feedstock hydrocarbon, resulting in a decrease in the octane number of the resulting product. Therefore, the usage of the light hydrocarbon is limited to petrochemical raw materials and raw materials for the production of city gas.
Various processes for producing aromatic hydrocarbons from a light hydrocarbon are known in which a light hydrocarbon is brought into contact with a gallium-containing crystalline silicate catalyst such as a crystalline gallosilicate (Japanese Patent Application Laid-Open Publication No. 59-98020), a crystalline galloaluminosilicate (Japanese Patent Application Laid-Open Publication No. 60-501357), or a hydrogen-type crystalline aluminogallosilicate of MFI structure (Japanese Patent Application Laid-Open Publication No. 62-254847).
However, these catalytic reforming processes are large endothermal reactions and thus fail to proceed with the reaction because the reaction temperature decreases unless the reaction layers are efficiently supplied with heat, resulting in a problem that aromatic hydrocarbons can not be produced at a high yield.
For catalytic reforming of a hydrocarbon fraction within the gasoline boiling point range with a conventional platinum/alumina catalyst in the presence of hydrogen, a process is known in which a plurality of catalyst reaction layers are arranged in series and heating means are arranged therebetween (U.S. Pat. No. 3,992,465). However, this catalytic reforming of a gasoline fraction is different in the composition of the feedstock and catalyst system from the above-described processes for the production of aromatics from a light hydrocarbon. Further, this patent document does not disclose the volume distribution of the plurality of reaction layers or the composition of the product outflowing from each of the reaction layers or suggest that the yield of the aromatics in the converted reaction product is significantly increased depending on the selection of the distribution or composition. Japanese Patent Application Laid-Open Publication No. 11-172261 discloses a catalytic reforming process for increasing BTX by providing two reforming zones. However, this process is complicated because different types of catalysts are used in those reforming zones.
As described above, for the production of aromatic hydrocarbons from a light hydrocarbon containing mainly hydrocarbons having 7 or fewer carbon atoms, a process wherein aromatic hydrocarbons are produced at a high yield has not been known yet.